1. Field of the Invention
The present invention relates generally to light emitting diode (LED) based lighting apparatus, and more particularly to an apparatus for driving an LED based lighting apparatus using high input voltage.
2. Description of Related Arts
LEDs are semiconductor-based light sources often employed in low-power instrumentation and appliance applications for indication purposes in the past. The application of LEDs in various lighting units has also become more and more popular. For example, high brightness LEDs have been widely used for traffic lights, vehicle indicating lights, and braking lights. In recent years, high voltage LED-based lighting apparatus have been developed to replace the conventional incandescent and fluorescent lamps.
An LED has an I-V characteristic curve similar to an ordinary diode. When the voltage applied to the LED is less than a forward voltage, only very small current flows through the LED. When the voltage exceeds the forward voltage, the current increases sharply. The output luminous intensity of an LED light is approximately proportional to the LED current for most operating values of the LED current except for the high current value. A typical driving device for an LED light is designed to provide a constant current for stabilizing light emitted from the LED and extending the life of the LED.
In order to increase the brightness of an LED light, a number of LEDs are usually connected in series to form an LED-based lighting string and a number of LED-based lighting strings may further be connected in series to form a lighting apparatus. The operating voltage required by each lighting string typically is related to the forward voltage of the LEDs in each lighting string, how many LEDs are employed for each of the lighting string and how they are interconnected, and how the respective lighting strings are organized to receive power from a power source.
Accordingly, in many applications, some type of voltage conversion device is required in order to provide a generally lower operating voltage to one or more LED-based lighting strings from more commonly available higher power supply voltages. The need of a voltage conversion device reduces the efficiency, costs more and also makes it difficult to miniaturize an LED-based lighting device.
In order to increase the efficiency and miniaturize the LED-based lighting apparatus, many techniques have been developed for the apparatus to use operating voltages such as 120V AC or 240V AC without requiring a voltage conversion device. In general, the LEDs in the apparatus are divided into a number of LED segments that can be selectively turned on or off by associated switches or current sources, and a controller is used to control the switches or current sources as the operating AC voltage increases or decreases.
In the prior arts, most of the high voltage LED-based lighting apparatus rely on the detection of the voltage level of the input AC voltage or the current flowing through the apparatus so as to control the switches or current sources to turn on or off selected LED segments. For example, U.S. Pat. Nos. 6,989,807 and 8,324,840 and U.S. Pat. Publication No. 2011/0089844 use a global controller that detects the input voltage level for controlling the current sources or switches connected to the LEDs. U.S. Pat. Publication No. 2012/0056559 and 2012/0217887 use a global controller to control current clamping units or switches according to local current sensing data.
As more and more LED-based lighting apparatus are used in high brightness lighting equipment with high input voltage, there is a strong need to design methods and apparatus that can drive and connect the LED-based lighting strings intelligently and efficiently to increase the utilization of the LEDs, reduce power loss and provide stable and high brightness by using the readily available AC source from a wall power unit.